Cavitation is a general term used to describe the behavior of voids or bubbles in a liquid. Cavitation occurs when water pressure is lowered below its vapor pressure or vapor pressure is increased to water pressure. When this happens, the water vaporizes, typically forming small bubbles of water vapor. But these bubbles of water vapor are typically not sustainable. Rather, the bubbles collapse, and when they do, they force liquid energy to very small volumes. This results in localized high temperature and the generation of shock waves.
Cavitation is ordinarily an unintended and often undesirable phenomenon. The collapse of small bubbles produces great wear on pump components and can dramatically shorten the useful life of a propeller or pump. It also causes a great deal of noise, vibration, and a loss of efficiency.
But the phenomenon of cavitation is not always undesirable; an exception is the phenomenon of “supercavitation.” In supercavitation, a sustainable bubble of gas inside a liquid is created by a “nose cavitator” of a moving object. This bubble envelopes the entire moving object except for the nose, with the result that the drag experienced by the moving object is significantly reduced. As a consequence, a supercavitating object can travel at far greater speeds for a given amount of thrust than an object that is moving in a conventional manner through water. Supercavitation enhances motion stability of an object as well.
A supercavitating (hereinafter also “cavity-running”) object's main features are a specially shaped nose and a streamlined, hydrodynamic, and aerodynamic body. When the object is traveling through water at speeds in excess of about one hundred miles per hour, the specially-shaped nose deflects the water outward so fast that the water flow separates and detaches from the surface of the moving object. Since water pressure takes time to collapse the wall of the resulting cavity, the nose opens an extended bubble or cavity of water vapor. Given sufficient speed, the cavity can extend to envelop the entire body of the object. A cavity-running object quite literally ‘flies’ through the surrounding gas. In the absence of sustaining propulsion, the moving object loses supercavitation and eventually stalls due to drag.